1. Field of the Invention
The invention relates in general to a flexible printed circuit board (PCB) and the method of fabricating the same, and more particularly to an identifiable flexible PCB and a method of fabricating the same.
2. Description of the Related Art
The computer printer, particularly the color inkjet printer, has become a necessity in the modern life. It can be used for printing out the desired information such as documents, color pictures, and photos. Generally, in the inkjet printer for color printing two cartridges are used. One is the black cartridge stored black ink, and the other is the color cartridge stored color ink. A flexible printed circuit board (PCB) disposed to the cartridge serves as a medium to lead the driving current to the printhead chip. After receiving the driving current, the printhead chip makes the cartridge to expel ink. Also, the flexible PCB comprises numerous holes, and the dimples of the printer circuit electrically contact with the conductive traces of the flexible PCB through the holes.
In the commercial inkjet printer market, there are some color-separate inkjet cartridges filled with black ink or color ink, such as cyan, magenta, yellow, light black, light cyan, light magenta, and light yellow inks. However, the colors of the related flexible PCBs are the same, and the other features such as the size, shape, and the pattern of conductive trace are very similar. It is difficult for the on-line operators to assemble the right flexible PCBs to the corresponding cartridges. To increase the assembly speed and accuracy, identifiable marks, such as a brand name or trademark, serial number, or tape specification, are printed on the flexible PCBs for identification. For example, the brand name or trademark provides identification for the manufactures and consumers. The serial number or tape specification provides identification for the on-line operators during assembling. Nevertheless, the extra printing procedure required in the fabrication process will increase the production cost. Also, the identifiable marks printed on the flexible PCBs could fade or peel off if exposed to a humid environment during a long period of time. Thus, printing the identifiable mark on the flexible PCB is not an economical and practical solution.
The flexible PCB is commonly fabricated by tape automated bonding (TAB) technique. Etching and punching are two typical TAB manufacturing processes. Etching process is characterized by etching the tape while the punching process is characterized by punching the tape to form the holes of the flexible PCB. The details are illustrated in the following description.
FIG. 1A˜FIG. 1J illustrate the conventional etching process for forming the holes of the flexible printed circuit board. First, a substrate 102 such as the material of polyimide (PI) is provided, as shown in FIG. 1A. A copper film 104 is then formed over the substrate 102 by sputtering, as shown in FIG. 1B. Two layers of photo-resist (PR) 106 are formed respectively on the bottom side of the substrate 102 and over the copper film 104, as shown in FIG. 1C. After exposing (FIG. 1D) and developing (FIG. 1E) the two layers of PR 106, the predetermined patterns of the holes and the conductive traces are defined. Next, a copper layer 108 is plated on the exposed copper film 104, and the copper layer 108 and the copper film 104 are integrated as a whole, as shown in FIG. 1F. Then, the substrate 102 is partially etched to form the holes 110, as shown is FIG. 1G. The remaining photo-resist 106 on the both sides are then removed, as shown in FIG. 1H. The copper layer 108 and the copper film 104 are exposed to air, and several gaps 109 are formed on the copper layer 108.
Then, the portion of the copper film 104 not covered by the copper layer 108 is removed by photolithography through the steps of coating a layer of PR, exposing, developing and etching. The conductive traces 111 are therefore formed by the combination of the remaining copper film 104 and the copper layer 108, as shown in FIG. 1I. Finally, the conductive trace 111 is enclosed by an insulation layer 112 for the purpose of protection, and the flexible PCB 114 is finished as shown in FIG. 1J. The conventional etching process has several drawbacks, including large time-consumption and low yield rate (high proportion of low-quality product). Also, it produces a sticky precipitate and discharges large amounts of wastewater during etching process. The cost of wastewater treatment is a considerable issue.
FIG. 2A˜FIG. 2I illustrate the conventional punching process for forming the holes of the flexible printed circuit board. First, a substrate 202 is provided (FIG. 2A), and an adhesive layer 204 is coated thereon, as shown in FIG. 2B. Then, the substrate 202 coated with the adhesive layer 204 is punched by the punching tool to form several holes 206, as shown in FIG. 2C. A copper layer 208 is further attached over the adhesive layer 204, as shown in FIG. 2D. Next, a photo-resist layer 210 is formed on the copper layer 208, followed by exposure and development. Numerous gaps 211 are created so as to expose parts of the copper layer 208 to the air, as shown in FIG. 2G. Then, portions of the copper layer 208 not covered by the PR layer 210 are etched, as shown in FIG. 2H.
Afterward, the remaining PR layer 210 is removed, and the discrete copper layer 208 (FIG. 2I) therefore forms the conductive traces. Finally, the conductive trace is enclosed by an insulation layer 212 for the purpose of protection, and then the flexible PCB 214 is completed as shown in FIG. 2I. Compared with the etching process mentioned above, the punching process is a short-term procedure requiring less cost and no wastewater treatment. However, the interval between adjacent holes is too wide to form a large number of holes of the flexible PCB. Consequently the contact area between the printer and the flexible PCB is decreased, and dramatically affects the precision of contact. Furthermore, the punching step could easily cause the breakage of the substrate, thus reducing the yield rate and increasing the cost of production.